1. Field of the Invention
The present invention relates to a developing apparatus which is applied to an image forming device such as a two-color copying machine.
2. Discussion of Background
Development of color versions has recently been promoted in the field of copying machines. For example, two-color copying machines have been developed for practical use which can produce color images in some other color than black. The developing apparatus applied to the color copying machine of this type includes two developing units facing a photosensitive drum (an image carrier). For example, a first developing unit can contain a red toner, and a second developing unit can contain a black toner. U.S. Pat. No. 4,466,379 (Nishimura) shows developing apparatus of this type. In this developing apparatus, the first developing unit comprises a first magnet roller arranged close to the photosensitive drum and a first sleeve rotatively arranged on the outer peripheral surface of the first magnet roller. The first magnet roller includes a first main pole and a pair of first convey poles sandwiching the first main pole. The first magnet roller is coupled to a first drive mechanism which rotates it through about 180.degree. in the clockwise and counterclockwise directions. The first sleeve is made of a nonmagnetic material and spaced from the photosensitive drum by a predetermined distance. The first sleeve is coupled to a first rotary mechansim. The second developing unit comprises a second magnet roller arranged close to the photosensitive drum and a second sleeve rotatively arranged on the outer peripheral surface of the second magnet roller. The second magnet roller includes a second main pole and a pair of second convey poles sandwiching the second main pole. The second magnet roller is coupled to a second drive mechanism which rotates it through about 180.degree. in the clockwise and counterclockwise directions. The second sleeve is made of a nonmagnetic material and spaced from the photosensitive drum by a predetermined distance. The second sleeve is coupled to a second rotary mechanism. A red developer is contained in the first developing unit, and a black developer is contained in the second developing unit.
If the red developing operation for the electrostatic latent image formed on the photosensitive drum is selected, the first drive mechanism causes the first main pole of the first magnet roller to face the surface of the drum. The first sleeve is rotated by the first rotary mechanism, and the magnetic brush of the red developer is formed on the first sleeve. The magnetic brush of the red developer is brought into contact with the photosensitive drum. As a result, the electrostatic latent image on the drum is developed with the red developer. Prior to developing in red, the second main pole of the second developing unit in rotated to the opposite side away from the drum by the counterclockwise rotation of the second drive mechanism, and the second sleeve is at rest. Therefore, as the magnetic brush of the black developer is not formed, black developer is not brought into contact with the drum. Thus, only the red developing operation is performed.
If the black developing operation for the electrostatic latent image formed on the photosensitive drum is selected, the first main pole of the first magnet roller is rotated to the opposite side away from the drum by the counterclockwise rotation of the first drive mechanism. At the same time. the second drive mechanism causes the second main pole of the second magnet roller to face the surface of the drum. The second sleeve is rotated by the second rotary mechanism, and the magnetic brush of the black developer is formed on the second sleeve. The magnetic brush of the black developer is brought into contact with the photosensitive drum. As a result, the electrostatic latent image on the drum is developed with the black developer. Prior to developing in black, the first main pole of the first developing unit is rotated to the oppsoite side away from the drum by the counterclockwise rotation of the first drive mechanism, and the first sleeve is at rest. Therefore, as the magnetic brush of the red developer is not formed, red developer is not brought into contact with the drum. Thus, only the black developing operation is performed.
However, in a conventional developing apparatus as described above, prior to developing in a first color, the second main pole of the second color developing unit must be rotated to the opposite side away from the photosensitive drum by the rotation of the second drive mechanism, and the second sleeve is at rest. Thus, there is no formation of the magnet brush of the second color developer. Therefore, the first and second developing unit require drive mechanisms such as reversible motors to rotate the first and second magnet rollers in the clockwise or counterclockwise direction. As a result, the parts of the developing apparatus increase in number, and the apparatus is expensive. Further, the developing apparatus requires control circuits to control the rotation in the clockwise or counterclockwise direction of these drive mechanisms. Therefore, the construction of the whole of the copying machine is complicated.